The burner must invariably be in position when a light is
applied, because, in the event of the pipes still containing an explosive
mixture, ignition would not be communicated through the small orifices of
the burner to the mixture in the pipes, and the application of the light
would not entail any danger of an explosion.
Gasfitters familiar with coal-gas should remember, when putting a system
of acetylene pipes into use for the first time, that the range over which
mixtures of acetylene and air are explosive is wider than that over which
mixtures of coal-gas and air are explosive, and that greater care is
therefore necessary in getting the pipes and rooms free from a dangerous
mixture.
The mains for very large installations of acetylene--_e.g._, for
lighting a small town--may advisedly be freed from air by some other plan
than simple expulsion of the air by acetylene, both from the point of
view of economy and of safety. If the chimney gases from a neighbouring
furnace are found on examination to contain not more than about 8 per
cent of oxygen, they may be drawn into the gasholder and forced through
the pipes before acetylene is admitted to them.
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